Chapter 39

1. Chapter 39 Plant Responses to Internal and External Signals

2. Overview: Stimuli and a Stationary Life • Linnaeus noted that flowers of different species opened at different times of day and could be used as a horologium florae, or floral clock • Plants, being rooted to the ground, must respond to environmental changes that come their way • For example, the bending of a seedling toward light begins with sensing the direction, quantity, and color of the light

3. Concept 39.1: Signal transduction pathways link signal reception to response • Plants have cellular receptors that detect changes in their environment • For a stimulus to elicit a response, certain cells must have an appropriate receptor • Stimulation of the receptor initiates a specific signal transduction pathway

4. Fig. 39-2 (b) After a week’s exposure to natural daylight. The potato plant begins to resemble a typical plant with broad green leaves, short sturdy stems, and long roots. This transformation begins with the reception of light by a specific pigment, phytochrome Before exposure to light. A dark grown potato has tall, spindly stems and nonexpanded leaves – morphological adaptations that enable the shoots to penetrate the soil. The roots are short, but there is little need for water absorp- tion because little water is lost by the shoots.

5. Fig. 39-3 CYTOPLASM CELL WALL Transduction Response 1 2 3 Reception Relay proteins and Activation of cellular responses second messengers Receptor Hormone or environmental stimulus Plasma membrane

6. Fig. 39-4-1 Transduction Reception 2 1 CYTOPLASM NUCLEUS Specific protein kinase 1 activated Plasma membrane cGMP Second messenger produced Phytochrome activated by light Cell wall Light • Internal and external signals are detected by receptors, proteins that change in response to specific stimuli. The light signal is detected by the phytochrome receptor, which then activates at least two signal transduction pathways.

7. Second messengers transfer and amplify signals from receptors to proteins that cause responses Transduction Reception 2 1 CYTOPLASM NUCLEUS Specific protein kinase 1 activated Plasma membrane cGMP Second messenger produced Phytochrome activated by light Cell wall Specific protein kinase 2 activated Light Ca2+ channel opened Ca2+ • One pathway uses cGMP as a second messenger that activates a specific protein kinase. The other pathway involves an increase in the cytosolic level of Ca2+, which activates a different protein kinase

8. Fig. 39-4-3 Transduction Reception Response 2 1 3 Transcription factor 1 CYTOPLASM NUCLEUS NUCLEUS Specific protein kinase 1 activated Plasma membrane cGMP P Second messenger produced Transcription factor 2 Phytochrome activated by light P Cell wall Specific protein kinase 2 activated Transcription Light Translation De-etiolation (greening) response proteins Ca2+ channel opened Ca2+ • Both pathways lead to exression of genes for proteins that function in the de-etiolation (greening) response.

9. Concept 39.2: Plant hormones help coordinate growth, development, and responses to stimuli • Hormones are chemical signals that coordinate different parts of an organism The Discovery of Plant Hormones • Any response resulting in curvature of organs toward or away from a stimulus is called a tropism • Tropisms are often caused by hormones • In the late 1800s, Charles Darwin and his son Francis conducted experiments on phototropism, a plant’s response to light • They observed that a grass seedling could bend toward light only if the tip of the coleoptile was present • They postulated that a signal was transmitted from the tip to the elongating region • http://www.youtube.com/watch?v=Ze8NV7cvW8k (phototropism)

10. Fig. 39-5a RESULTS Shaded side of coleoptile Control Light Illuminated side of coleoptile

11. Fig. 39-5b RESULTS Darwin and Darwin: phototropic response only when tip is illuminated Light Tip covered by opaque cap Site of curvature covered by opaque shield Tip removed Tip covered by trans- parent cap

12. Fig. 39-5c RESULTS Boysen-Jensen: phototropic response when tip is separated by permeable barrier, but not with impermeable barrier. Demonstrated that the signal was a mobile chemical substance Light Tip separated by gelatin (permeable) Tip separated by mica (impermeable)

13. Fig. 39-6 RESULTS Excised tip placed on agar cube • In 1926, Frits Went extracted the chemical messenger for phototropism, auxin, by modifying earlier experiments Growth-promoting chemical diffuses into agar cube Agar cube with chemical stimulates growth Control (agar cube lacking chemical) has no effect Offset cubes cause curvature Control

14. A Survey of Plant Hormones • In general, hormones control plant growth and development by affecting the division, elongation, and differentiation of cells • Plant hormones are produced in very low concentration, but a minute amount can greatly affect growth and development of a plant organ

15. Table 39-1

16. Auxin • The term auxin refers to any chemical that promotes elongation of coleoptiles • Apical meristems and young leaves are primary sites of auxin synthesis • The Role of Auxin in Cell Elongation • According to the acid growth hypothesis, auxin stimulates proton pumps in the plasma membrane • The proton pumps lower the pH in the cell wall, activating expansins, enzymes that loosen the wall’s fabric • With the cellulose loosened, the cell can elongate

17. Fig. 39-8 3 Expansins separate microfibrils from cross- linking polysaccharides. (activated by low pH) Cell wall–loosening enzymes Cross-linking polysaccharides Expansin CELL WALL 4 Cleaving allows microfibrils to slide. Cellulose microfibril H2O Cell wall Cell wall becomes more acidic. 2 Plasma membrane 1 Auxin increases proton pump activity. Nucleus Cytoplasm Plasma membrane Vacuole CYTOPLASM 5 Cell can elongate.

18. Cytokinins • Cytokinins are so named because they stimulate cytokinesis (cell division) • Control of Cell Division and Differentiation • Cytokinins are produced in actively growing tissues such as roots, embryos, and fruits • Cytokinins work together with auxin to control cell division and differentiation • Control of Apical Dominance • Cytokinins, auxin, and other factors interact in the control of apical dominance, a terminal bud’s ability to suppress development of axillary buds • If the terminal bud is removed, plants become bushier

19. Fig. 39-9 Lateral branches “Stump” after removal of apical bud (b) Apical bud removed Axillary buds (c) Auxin added to decapitated stem (a) Apical bud intact (not shown in photo)

20. Gibberellins • Gibberellins have a variety of effects, such as stem elongation, fruit growth, and seed germination • Stem Elongation • Gibberellins stimulate growth of leaves and stems • In stems, they stimulate cell elongation and cell division • Fruit Growth • In many plants, both auxin and gibberellins must be present for fruit to set • Gibberellins are used in spraying of Thompson seedless grapes

21. Fig. 39-10 (b) Gibberellin-induced fruit growth Gibberellin-induced stem growth

22. Fig. 39-11 Gibberellins (GA) send signal to aleurone. 1 Sugars and other nutrients are consumed. 2 3 Aleurone secretes -amylase and other enzymes. Aleurone Endosperm -amylase Sugar GA GA Water Radicle Scutellum (cotyledon) • After water is imbibed, release of gibberellins from the embryo signals seeds to germinate

23. Brassinosteroids • Brassinosteroids are chemically similar to the sex hormones of animals • They induce cell elongation and division in stem segments • (I have never seen this one on any AP exam.)

24. Abscisic Acid • Abscisic acid (ABA) slows growth • Two of the many effects of ABA: • Seed dormancy • Drought tolerance • Seed Dormancy • Seed dormancy ensures that the seed will germinate only in optimal conditions • In some seeds, dormancy is broken when ABA is removed by heavy rain, light, or prolonged cold • Drought Tolerance • ABA is the primary internal signal that enables plants to withstand drought

25. Fig. 39-12 Early germination in red mangrove Coleoptile Early germination in maize mutant (caused by a mutation)

26. Ethylene (gaseous hormone) • Plants produce ethylene in response to stresses such as drought, flooding, mechanical pressure, injury, and infection • The effects of ethylene include response to mechanical stress, senescence, leaf abscission, and fruit ripening • Senescence • Senescence is the programmed death of plant cells or organs • A burst of ethylene is associated with apoptosis, the programmed destruction of cells, organs, or whole plants • Leaf Abscission • A change in the balance of auxin and ethylene controls leaf abscission, the process that occurs in autumn when a leaf falls • Fruit Ripening (positive feedback) • A burst of ethylene production in a fruit triggers the ripening process

27. Fig. 39-15 0.5 mm Protective layer Abscission layer Stem Petiole

28. Concept 39.3: Responses to light are critical for plant success • Light cues many key events in plant growth and development • Effects of light on plant morphology are called photomorphogenesis • Plants detect not only presence of light but also its direction, intensity, and wavelength (color) • A graph called an action spectrum depicts relative response of a process to different wavelengths • Action spectra are useful in studying any process that depends on light

29. Fig. 39-16a 1.0 436 nm 0.8 0.6 Phototropic effectiveness 0.4 0.2 0 450 400 500 650 700 600 550 Wavelength (nm) Action spectrum for blue-light phototropism illustrates that only light wave- lenghts below 500 nm induce curvature

30. Fig. 39-16b Light Time = 0 min Time = 90 min (b) Coleoptile response to light colors. Phototropic bending toward light is controlled by phototropin, a photoreceptor sensitive to blue and violet light, particularly blue light

31. Fig. 39-16 1.0 436 nm 0.8 0.6 Phototropic effectiveness 0.4 0.2 0 500 400 550 450 650 700 600 Wavelength (nm) (a) Action spectrum for blue-light phototropism Light Time = 0 min Time = 90 min (b) Coleoptile response to light colors

32. Phytochromes as Photoreceptors • Phytochromes are pigments that regulate many of a plant’s responses to light throughout its life • These responses include seed germination and shade avoidance Phytochromes and Seed Germination • Many seeds remain dormant until light conditions change • In the 1930s, scientists at the U.S. Department of Agriculture determined the action spectrum for light-induced germination of lettuce seeds

33. Red light increased germination, while far-red light inhibited germination • The photoreceptor responsible for the opposing effects of red and far-red light is a phytochrome RESULTS Dark (control) Dark Far-red Dark Red Red Red Far-red Dark Red Red Far-red Far-red Red

34. Fig. 39-18 Structure of phytochrome Two identical subunits Chromophore Photoreceptor activity Kinase activity

35. Fig. 39-UN1 • Phytochromes exist in two photoreversible states, with conversion of Pr to Pfr triggering many developmental responses Red light (660 nm) Pr Pfr Far-red light (730 nm)

36. Fig. 39-19 Pfr Pr Red light Responses: seed germination, control of flowering, etc. Synthesis Far-red light Slow conversion in darkness (some plants) Enzymatic destruction Absorption of red light causes the Pr to change to Pfr. Far red light reverses this conversion. In most cases, it is the Pfr form of the pigment that switches on physiological and developmental responses in the plant

37. Biological Clocks and Circadian Rhythms • Many plant processes oscillate during the day • Many legumes lower their leaves in the evening and raise them in the morning, even when kept under constant light or dark conditions • Circadian rhythms are cycles that are about 24 hours long and are governed by an internal “clock” • Circadian rhythms can be entrained to exactly 24 hours by the day/night cycle • The clock may depend on synthesis of a protein regulated through feedback control and may be common to all eukaryotes

38. Sleep movement of a bean plant. The movements are caused by reversible changes in the turgor pressure of cells on opposing sides of the motor organs of the leaf. Continues even when leaves are “held down or up”. They return to “normal” position for that time of day. Noon Midnight

39. The Effect of Light on the Biological Clock • Phytochrome conversion marks sunrise and sunset, providing the biological clock with environmental cues Photoperiodism and Responses to Seasons • Photoperiod, the relative lengths of night and day, is the environmental stimulus plants use most often to detect the time of year • Photoperiodism is a physiological response to photoperiod

40. Photoperiodism and Control of Flowering • Some processes, including flowering in many species, require a certain photoperiod • Plants that flower when a light period is shorter than a critical length are called short-day plants (actually long night plants) • Plants that flower when a light period is longer than a certain number of hours are called long-day plants (actually short night plants) • Flowering in day-neutral plants is controlled by plant maturity, not photoperiod

41. Critical Night Length • In the 1940s, researchers discovered that flowering and other responses to photoperiod are actually controlled by night length, not day length • Short-day plants are governed by whether the critical night length sets a minimum number of hours of darkness • Long-day plants are governed by whether the critical night length sets a maximum number of hours of darkness • Red light can interrupt the nighttime portion of the photoperiod • Action spectra and photoreversibility experiments show that phytochrome is the pigment that receives red light

42. Fig. 39-21 24 hours (a) Short-day (long-night) plant Light Flash of light Darkness Critical dark period (b) Long-day (short-night) plant Flash of light

43. Fig. 39-22 24 hours R RFR RFRR RFRRFR Long-day (short-night) plant Short-day (long-night) plant Critical dark period

44. A Flowering Hormone? • The flowering signal, not yet chemically identified, is called florigen (Have not seen on ap exam) • Florigen may be a macromolecule governed by the CONSTANS gene

45. Fig. 39-23 24 hours 24 hours 24 hours Graft Short-day Plant (long night) Long-day Plant (short night) Long-day plant grafted to short-day plant

46. Concept 39.4: Plants respond to a wide variety of stimuli other than light • Because of immobility, plants must adjust to a range of environmental circumstances through developmental and physiological mechanisms Gravity • Response to gravity is known as gravitropism • Roots show positive gravitropism; shoots show negative gravitropism

47. Gravity • Response to gravity is known as gravitropism • Roots show positive gravitropism; shoots show negative gravitropism • Plants may detect gravity by the settling of statoliths, specialized plastids containing dense starch grains • http://www.youtube.com/watch?v=I7TmX0rrNRM

48. Fig. 39-24 Statoliths 20 µm (a) Root gravitropic bending (b) Statoliths settling

49. Mechanical Stimuli • The term thigmomorphogenesis refers to changes in form that result from mechanical disturbance • Rubbing stems of young plants a couple of times daily results in plants that are shorter than controls • Thigmotropism is growth in response to touch • It occurs in vines and other climbing plants • Rapid leaf movements in response to mechanical stimulation are examples of transmission of electrical impulses called action potentials • http://www.youtube.com/watch?v=8Xf5FsMESXQ (mimosa leaf)

50. Fig. 39-25 Altering gene expression by touch. The shorter plant on the left was rubbed twice a day. The untouched plant grew much taller.